procfs: use an enum for possible hidepid values

Previously, the hidepid parameter was checked by comparing literal
integers 0, 1, 2. Let's add a proper enum for this, to make the
checking more expressive:

0 → HIDEPID_OFF
1 → HIDEPID_NO_ACCESS
2 → HIDEPID_INVISIBLE

This changes the internal labelling only, the userspace-facing interface
remains unmodified, and still works with literal integers 0, 1, 2.

No functional changes.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Lafcadio Wluiki <[e-mail]>
Signed-off-by: Djalal Harouni <[e-mail]>
Acked-by: Kees Cook <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/proc/base.c

···
 	task = get_proc_task(inode);
 	if (!task)
 		return -ESRCH;
-	has_perms = has_pid_permissions(pid, task, 1);
+	has_perms = has_pid_permissions(pid, task, HIDEPID_NO_ACCESS);
 	put_task_struct(task);
 
 	if (!has_perms) {
-		if (pid->hide_pid == 2) {
+		if (pid->hide_pid == HIDEPID_INVISIBLE) {
 			/*
 			 * Let's make getdents(), stat(), and open()
 			 * consistent with each other.  If a process
···
 	stat->gid = GLOBAL_ROOT_GID;
 	task = pid_task(proc_pid(inode), PIDTYPE_PID);
 	if (task) {
-		if (!has_pid_permissions(pid, task, 2)) {
+		if (!has_pid_permissions(pid, task, HIDEPID_INVISIBLE)) {
 			rcu_read_unlock();
 			/*
 			 * This doesn't prevent learning whether PID exists,
···
 		int len;
 
 		cond_resched();
-		if (!has_pid_permissions(ns, iter.task, 2))
+		if (!has_pid_permissions(ns, iter.task, HIDEPID_INVISIBLE))
 			continue;
 
 		len = snprintf(name, sizeof(name), "%d", iter.tgid);

fs/proc/inode.c

···
 
 	if (!gid_eq(pid->pid_gid, GLOBAL_ROOT_GID))
 		seq_printf(seq, ",gid=%u", from_kgid_munged(&init_user_ns, pid->pid_gid));
-	if (pid->hide_pid != 0)
+	if (pid->hide_pid != HIDEPID_OFF)
 		seq_printf(seq, ",hidepid=%u", pid->hide_pid);
 
 	return 0;

fs/proc/root.c

···
 		case Opt_hidepid:
 			if (match_int(&args[0], &option))
 				return 0;
-			if (option < 0 || option > 2) {
+			if (option < HIDEPID_OFF ||
+			    option > HIDEPID_INVISIBLE) {
 				pr_err("proc: hidepid value must be between 0 and 2.\n");
 				return 0;
 			}

include/linux/pid_namespace.h

···
 
 struct fs_pin;
 
+enum { /* definitions for pid_namespace's hide_pid field */
+	HIDEPID_OFF	  = 0,
+	HIDEPID_NO_ACCESS = 1,
+	HIDEPID_INVISIBLE = 2,
+};
+
 struct pid_namespace {
 	struct kref kref;
 	struct pidmap pidmap[PIDMAP_ENTRIES];